Continuous ink jet printers are well known in the field of coding and industrial marking of various products, for example to mark barcodes or the expiration date on food products directly on the production line and at a high speed rate. This type of printer is also found in certain decorative fields where the graphic printing possibilities of the technology are exploited.
It is traditionally distinguished two categories within continuous ink jet printers:                on one hand, multi-deflection continuous jet printers where each drop of a single jet (or few jets) can be sent on various paths corresponding to controls for different deflections of the drops, thereby achieving raster scans of the zone to be printed following a scan direction which is the deflection direction;        on the other hand, binary continuous jet printers where a plurality of jets placed side by side each have only one drop path designed for printing; the synchronous control, at a given moment, of all of the jets makes it possible to print on the medium according to a pattern corresponding in general to that of the nozzles on the nozzle plate.        
In both types of printers, the other direction perpendicular to scan raster of the zone to be printed is covered by relative movement between the printing head and the medium to be printed.
As illustrated in FIG. 1, these printers include a printing head, generally distant from the body of the printer; it is connected thereto by an umbilical bringing the hydraulic and electrical feeds necessary for the operation of the head.
The head 101 has a drop generator 102 supplied with pressurized electrically conductive ink and capable of emitting one or several continuous jets 109 through calibrated nozzles 105, the jets being transformed into a succession of drops under the action of a periodic stimulation system situated upstream from the nozzle(s), from a point called the “break up point” 106 where the drops are formed.
When the drops are not intended for printing, they are directed toward a gutter 103 which recovers them and returns them toward an ink circuit 100 in order for the ink to be recycled.
Devices placed along the jet (charge 107 and deflection 104 electrodes) make it possible, upon command, to electrically charge and deflect the drops; these drops are deviated from their natural ejection trajectory from the drop generator.
The drops 108 intended for printing escape the gutter and are deposited on the medium to be printed. More specifically, a charge electrode 107 is designed to selectively charge each of the drops formed with a predetermined electrical charge value. To do this, the ink being kept at a fixed electrical potential in the drop generator, a determined voltage is applied to the charge electrode 107 which is different at each period of drop formation. Thus, by electrostatic effect, a determined quantity of electrical charges is taken on by each drop at the moment when it detaches from the jet. Downstream from the charge electrode 107, it is advantageously possible to provide a device making it possible to measure the electrical charge actually taken on by each drop as well as its speed in the head.
A set of deflection electrodes 104, in the form of plates, is placed on either side of the trajectory of the drops downstream from the charge electrode 107. These two plates are brought to a high fixed relative potential producing an electrical field Ed essentially perpendicular to the trajectory of the drops, capable of deflecting the electrically charged drops which engage between the plates 104.
The amplitude of the deflection depends on the charge and the speed of these drops. These deflected trajectories 108 escape the gutter 103 in order to impact the medium to be printed.
Ink jet printers also comprise a fluid circuit 100 which performs the two basic functions, i.e. providing ink to the drop generator 102 at a suitable pressure and with a suitable quality, and recovering, by suction, the ink not used for printing from the jets. The fluid circuit 100 is connected on one hand to a removable ink cartridge 30 and on the other hand to a removable solvent cartridge 40, the solvent making it possible to adjust the viscosity and/or concentration of the ink intended for printing.
Ink jet printers also comprise a controller 200. This controller 200 interacts on one hand directly with the drop generator 102 and the charge electrodes 107 in order to stimulate the inkjet and manage the printing sequencings, and on the other hand with the fluid circuit 100, in order to manage the action sequencings and perform the processing enabling the activation of the different functions of the fluid circuit 100. The printing sequencings consist of generating the succession of voltages synchronized with the formation of drops making it possible to charge each of the drops according to the pattern to be printed. The action sequencings of the fluid circuit consist of controlling the ink pressure in order to adjust the speed of the drops, carrying out the measurements on the sensors, driving the active components (solenoid valves, pump motors).
The controller is also connected with the production line of the goods to be printed, which provides it with the temporal information allowing it to synchronize the printing of messages with the passage of the products under the head. This information allows it to measure the linear speed and the throughput rate of each production line.
Inkjet printers lastly comprise an interface 300 interacting with the controller 200 which gives the user (operator) a means to drive the printer and in return to be informed of the operation thereof. Depending on the different technologies used over time, the interfaces have been able to assume different forms such as, for example, having control buttons or keyboards, indicator lights, displays or screens which are more or less sophisticated, and, possibly, electrical or computer connections allowing remote control of the printer. This being the case, the interface 300 of the printer allows the final user (operator) to have several differentiated operating modes, in particular:                a maintenance mode making it possible to physically put the printer in its condition to print, but without carrying out the production;        a production preparation mode making it possible to generate the data to print and configure the printing to be done for production;        a production printing mode where the state of the printer and the production monitoring are shown while the data (pattern) is printed on demand (from the production line or signals internal to the printer).        
During the production of products, any untimely stopping of a continuous production line, in particular in a line at a high rate, is very detrimental (loss of exploitation, discarding of non-compliant products). Thus, preventive maintenance of parts or sub-assemblies of the line is provided to avoid any untimely stops.
One can consider that the untimely stops of one or several continuous inkjet printers integrated in a continuous production line are due mainly to the degradation of the printing quality and the exhaustion of one of the consumable fluids (ink or solvent). Indeed, stops due to operational breakdowns are rare as they can often be avoided by preventive maintenance of the printer.
The degradation of the printing quality, to the point of becoming unacceptable, is caused mainly by the progressive dirtying of the printing head.
In order to minimize or delay dirtying of the head in continuous inkjet printers, it is known to perform on one hand, a continuous pressurization of the inside of the head and on the other hand, preventive interventions such as cleaning of all or part of the components of the head (drop generator, nozzle, charge electrodes, deflection electrodes and gutter) and making optimal adjustments thereof.
Exhaustion of the consumable fluids generally leads to automatically stopping of the concerned printer by itself. Indeed, in the contrary case, either the printing head could ingest air in case of the absence of ink entering from the supply pump, or the jet(s) emitted by the head could no longer be controlled due to the deterioration of the ink quality which would no longer be controlled due to the lack of solvent. Thus, not stopping the printer under these conditions would require a long intervention to return the printer to its condition to print correctly, which would thereby penalize the availability of the continuous production line.
Thus, many continuous inkjet printers according to the prior art implement solutions to anticipate the exhaustion of the consumable fluids (ink and solvent).
By inventorying commercial solutions and solutions described in the literature, the inventors came to the conclusion that there are, to date, two categories of solutions for anticipating the exhaustion of consumable fluids (ink and solvent) in a continuous inkjet printer:
1/the possibility for the user (operator) of being able to resupply the printer with consumables during production, either by replacing removable cartridges or by filling fixed tanks inside the printer from transfer containers at the disposal of the user;
2/indicators, as components of the user interface, of the level or volume of consumable fluids remaining in the printer indicating the approach of the exhaustion of the consumables. These indicators are connected in input to systems for determining the quantity of ink and/or solvent and are often connected at their output to alarms, other components of the user interface, triggered to warn the user of an exhaustion threshold. Therefore, at best, the user, notified by the interface of the printer, can resupply the printer during production. The user interfaces according to the prior art have as components, alarms for detecting overly high levels and/or indicators of evaluated volumes of consumables in the form of a proportion (percentage) relative to the initial contents of the tanks.
The systems for determining the quantity of ink and/or solvent used in the inkjet printers according to the prior art implement solutions consisting of detecting levels of fluids in the tanks.
One of the most reliable and easiest to implement, which is used for example in Series S8 type printers by the company Imaje, uses the principle of rod level sensors dipping into the tank: the fluid resistivity is measured between two rod level sensors, and if the ink short circuits the rods, the drop in resistivity is detected to declare a presence of ink at that level.
This system remains, however, costly due to the electronic protections which the standards require be implemented when electrical currents pass in flammable environments or fluids, which is in general the case of ink with volatile solvent. It must be noted that this type of detector by rod level sensor cannot be used with insulating fluids as solvents generally are. The solvent level is then not really detected and it is only through the degradation of the ink quality for lack of solvent that the printer notifies the user of the exhaustion of the solvent. There are, of course, other devices known by those skilled in the art making it possible to detect a fluid level such as capacitive, optical or other sensors, but the device must be explosion-proof given the flammable nature of the fluids used.
One can also cite the solution disclosed in application WO2009/047497 by the company Videojet which consists of evaluating the quantity of fluid remaining in a removable semi-rigid sealed cartridge.
The measuring system includes means for measuring the level of the vacuum created by the withdrawal of the consumable fluid which progressively deforms the cartridge, this vacuum value being representative of the quantity of remaining fluid. This measurement can only be approximate and concerns only the fluids contained in cartridges of new consumable fluids, i.e. which are not present in the ink circuit itself.
One can also cite the solution disclosed in application WO2007/129110 by the company Domino, which consists of determining the quantity of consumables remaining from the initial quantity of the reserves and a continuous evaluation of the fluid consumption. Thus, for the solvent, the number of doses of solvent used to correct the viscosity of the ink or for cleaning is counted. For ink, the number of printed positions of drops is counted, from the decomposition of patterns to be printed messages and characters). These volume evaluations are very imprecise because the volumes of the doses of solvent or drops of ink printed are not known with sufficient precision (and can also vary, depending on external conditions), and likewise the number of drops actually printed is not precisely known.
Thus, although many solutions for anticipating the exhaustion of consumable fluids (ink and solvent) exist in the prior art, the situation remains imperfect and restrictive for a user (operator) of continuous inkjet printers in an industrial setting. Indeed:                the continuous inkjet printers of the prior art do not offer the possibility of precisely determining the autonomy in consumable fluids: indeed, they do not have systems for precisely measuring the quantity of consumable fluids (ink and solvent) still available, or systems for precisely measuring the actual consumption of consumable fluids in a given production sequence;        the user (operator) interface of the printer does not provide the best information to facilitate management of consumables by the user: the indication of a discrete level or a volume of consumable in the form of a percentage of an initial capacity does not allow him to easily determine whether this quantity will be sufficient for a given production duration or quantity of products to be marked in light of the preceding, i.e. the imprecise measurement of the quantity of consumable fluids and the actual consumption of the consumable fluids in a given production sequence. It is therefore essential for the operator to dedicate part of his attention to regularly monitoring the printer's level of consumables;        furthermore, the operator of a production line is not necessarily available to take care of the printer when the alarms are triggered. An alarm is therefore intrusive and can lead to a stressful situation generating errors;        the alarms are triggered, in general with a safety margin corresponding to a minimal volume of consumable material still available; either the operator has the time to resupply the printer right away at the risk of wasting consumable product, which is often costly, as the cartridges to be changed are not completely empty yet, or he must monitor the evolution of the ink and/or solvent consumption for subsequent intervention when the cartridge(s) is/are completely empty.        
An object of the invention is therefore to overcome all or some of the aforementioned drawbacks.
One aim of the invention is therefore to propose a system for determining the autonomy in fluids (ink or solvent) of a continuous inkjet printer which is precise.